Adult hematopoiesis originates from primitive pluripotent stem cells in the bone marrow. In vitro expansion of hematopoietic stem cells is a major goal of experimental and clinical hematology. In fact, gene therapy of primitive hematopoietic cells has been hindered by the inability to routinely expand stem cells in culture. The objective of this proposal is to develop a new and novel screen that will identify factors that cause bone marrow stem cells to self renew. In phase I, we will construct combinatorial libraries, and identify ligands specific to the surface of primitive human and murine hematopoietic stem cells and murine stem cell lines. Following this, we will design and perform high throughput screens to identify mimetic molecules that behave as agonists in proliferation of hematopoietic stem cell lines. In phase II, individual candidates will be tested in a variety of assays on primary human primitive hematopoietic cells. These assays should distinguish true stem cell renewal from proliferation in conjunction with lineage commitment. PROPOSED COMMERCIAL APPLICATIONS: Agonists identified in phase I will be provided to the medical and research community in the form of reagents for gene therapy to expand stem/progenitor cells in vitro, to improve transduction of hematopoietic stem/progenitor cells by retroviral vectors for sustained therapeutic benefit, to improve blood cell production in vivo for cost-effective therapeutics in myelosuppression and for dose-intensification treatment of various diseases, as reagents for immunodiagnostics, cell separation, and research on fundamental mechanisms in hematopoiesis. The libraries should also be a rich, new, inexpensive source of markers for stem and progenitor cell separation.